Usually both play a part. White fabrics look grey when they pick up soil, oils and loose dyes from the wash water and from other garments, a process often called redeposition. They can also lose some of the whiteness the mill gave them in finishing, since many white fabrics are treated with optical brightening agents that make them look brighter. Washing at low temperatures, overloading the machine, using too little detergent, and washing whites with coloured items all make greying more likely.

The fabric side

Fabrics differ in how white they are to start with and how well they keep it. Polyester and blends can hold oily soil more than cotton, and some finishes wash out faster than others. Compare your fabric with a competitor's after the same washing: wash both with a named procedure such as ISO 6330 for several cycles, together with a standard soiled or coloured load if you want to imitate real use, and compare whiteness side by side under the same light.

Colour transfer

If undershirts are washed with dark items, dye from those items can stain them. Colour fastness to washing, for example with ISO 105-C06, includes a staining assessment on adjacent fabrics, which shows how much a coloured item stains white fabric. Your own dark products should be tested so they do not cause the problem for customers.

Care label and advice

The FTC Care Labeling Rule, 16 CFR Part 423, requires care instructions for regular care and warnings about procedures that consumers could reasonably be expected to use that would harm the product. Many brands add "wash with like colours" for whites. Product pages can add simple advice: wash whites separately, do not overload the machine, use the recommended amount of detergent and the warmest temperature the label allows.

What to specify

Ask the mill to state the whiteness finish and to keep it consistent across lots. Keep the approved white fabric as a reference, and compare bulk against it before and after washing.

If the problem continues, look at a different fabric quality or blend rather than only at care advice.